Serious Games represent an important opportunity for improving education thanks to their ability to compel players and to present realistic simulations of real-life situations. The scientific community is aware that we are just at the beginning of a proper use of gaming technologies for education and training and, in particular, there is a need for scientific and engineering methods for building games not only as more realistic simulations of the physical world, but as means that provide effective learning experiences. This requires an ever closer cooperation among the various actors involved in the overall SG life-chain, putting pedagogy in a central role, given the educational target of the SGs. This paper addresses the till-now inadequate integration of educational and game design principles and proposes techniques, methods and mechanisms that allow designers with different background to dialogue among each other and to define games that are able to integrate - by design - entertainment and educational features. In particular, the paper follows a design path that starts from the definition of reference frameworks and then analyses the typical categories of design patterns, before focusing on the user-interaction modalities - seen from a pedagogical point of view - given their relevance for the end-users. In the end, we discuss the sandbox serious game model, that looks suited to implement joint pedagogical and entertainment features. We believe that the indications provided in this paper can be useful for researchers and stakeholders to understand the typical issues in SG design and to get inspiration about possible solutions that take into account the need to implement tools that are effective both as an entertainment medium and as an education tool.